Gage Blocks (10010C-10015C)

This calibration service provides for measurement of the length of gage blocks by either mechanical comparison to NIST master gage blocks or by direct interferometry. Generally, the length is transferred from known master blocks by a systematic intercomparison procedure using mechanical comparators. As a consequence of limited resources, interferometric measurements are limited to no more than 25 blocks per order.

All gage blocks submitted for test should be in substantially new block condition, and each block should be marked with an identification number. In shipping gage blocks, extreme care should be taken to prevent both corrosion and damage by contact with other gage blocks. All defining steel surfaces should be greased and padded with waxed paper or volatile rust inhibitor treated paper. A greased steel surface coming in contact with newspaper, wrapping paper (unwaxed), or excelsior is likely to corrode. Sets of gage blocks should have packing inside the case and the case should be bound shut, since the clasps on cases frequently open or break during shipment.

Square or rectangular blocks up to 500 mm (20 inch) long are routinely calibrated, and the lengths reported in English or metric units. For blocks longer than 500 mm (20 inch), the NIST technical contact should be consulted in advance to discuss alternative measurement techniques, approximate costs, and scheduling.

The reported measurement uncertainty depends on the length of the blocks. For blocks near 2 mm (0.1 inch), the expanded uncertainty is approximately 25 nm. The uncertainty is larger for blocks significantly longer or shorter than 2 mm. A report describing the NIST gage block laboratory measurement assurance process is sent to all new customers and on request to past customers.

Line Standards (10020C-10025C)

Line scales are calibrated in the NIST Line Scale Interferometer (LSI). This instrument consists of a scanning electro-optical line detector, a high precision one-axis motion system, and a high accuracy heterodyne interferometer for determining the displacement of the test artifact beneath the line detector. The wavelength of a stabilized helium-neon laser corrected for the refractive index of the air is used as the metric for displacement measurement. The instrument is housed in a temperature controlled chamber in which all environmental properties are carefully monitored.

With this instrument a minimum line spacing of 2 µm and a maximum line spacing of 1 m can be measured on a line scale. Line widths from 1 µm to 100 µm can be accommodated, with 2 µm to 10 µm being optimum. The maximum overall length of a line standard should not exceed 1020 mm, and the maximum width of any part of the scale should not exceed 150 mm. All scale graduation line intervals are generally measured from line center to line center. In order to get repeatable measurements the artifact needs to be supported in the same manner as it was at NIST.

The accuracy of the calibration depends significantly on the shape and optical properties of the graduation lines, the optical properties of the background, and the flatness of the graduated surface. The accuracy of the calibration will be near optimum if the graduation lines have straight and smooth edges and are between 2 µm to 10 µm wide and are 20 µm to 100 µm long; if there is good contrast between the graduation lines and the background; and if the graduated surface of the artifact is flat enough, approximately 1 µm, to stay in focus when mounted for measurement in the LSI. Expanded uncertainties of 3 nm or less can be achieved for intervals of 10 mm or less. For intervals of about 1 m, expanded uncertainties of 100 nm or less can be attained.

The calibration of metal tapes and scales is carried out in a laboratory that houses two permanent working standards, a laser-based displacement interferometer and a 60 m (200 ft) stainless steel bench. Measurements are primarily performed using the laser-based displacement interferometer system. A measuring carriage consisting of a cube corner reflector and an attached microscope is used to manually locate the scale graduations. The laboratory is maintained at 20 °C ± 0.25 °C but a control system can vary the chamber temperature over the range of 15 °C to 22 °C for special tests

Calibration of metal tapes is made with the tape under tension and supported on a horizontal flat surface. Tapes less than or equal to 60 m in length typically include seven measurement points. The location of these measurement points must be specified and included on a calibration purchase order. The default tape calibration includes seven equally spaced points, which includes a measurement of the full length of the tape.

Some professional organizations, such as the American Petroleum institute, publish Standards that require specific measurement points. It is the customer's responsibility to communicate the required measurement points or cite the specific published Standard that describes the requisite measurement points

On request, each interval that is calibrated on a surveying tape can have centenary corrections computed for lengths composed of two (single catenary), three, four, and five equidistant points of support

Calibration of metal scales is made with the scale supported on a horizontal flat surface

A laser-based displacement interferometer standard is normally used for calibrating all tapes and scales. Under ideal conditions, steel tapes can be calibrated with an expanded uncertainty of (k=2) of (3.4 x 10-4 + 3.2 x 10-6L ) ft, where L is the length of the tape in feet. Calibrations made with respect to the stainless steel tape bench have an associated expanded uncertainty (k=2) of 3.4 x 10-4 + 10 x 10-6L .

A NIST serial number will be engraved on each calibrated tape and scale for identification purposes.

Upon request and by particular arrangement, NIST is capable of performing special tests of a variety of long length artifacts. These tests are limited only by the breadth and weight of the artifact proposed for testing.

The technical contact(s) should be consulted before the long length artifact tests are requested.

NIST provides special tests for measurements on end standards with spherical ends, gage blocks of unusual shape or exotic materials, or measurement of standards between 600 mm (24 inch) and 6 m (240 inch). However, the technical contact(s) must be consulted before this special test is requested.

It is a special test when customer requests characterization in inches rather than SI units.